3 Answers
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As the name suggests, they direct the fast-moving and hot air from the jet engines upwards. This protects the personnel and equipment behind the departing aircraft from getting blown around or damaged from the jet blast. Because of the high temperature of the jet blast, they may have active cooling systems, and must be allowed to cool sufficiently before being used as regular deck again.

To illustrate the risks, see this video, which shows a vehicle being blown away by a 747.

There was also a recent event in Pakistan where a 737 damaged the tarmac (and, as a result, itself) during an engine run-up.

To make a comparison, each engine on a 737-400 can produce up to 22,000 lbf (98 kN) of thrust. The Su-33 in your picture has two engines close together which can each produce 16,750 lbf (74.5 kN) of thrust, or 28,214 lbf (125.5 kN) with afterburner. That's a total of 56,248 lbf of thrust in afterburner, or 2.5 times the force of one engine on the 737-400. Of course, this does not directly equate to jet blast or temperatures, but it is a good indicator of the forces involved.

To provide a better idea of the jet blast, see this chart (based on the 737-800). At 20 m behind the tail (which is about 20 m behind the engines), the jet blast can reach 300 km/hr at takeoff thrust. For more details about jet blast on the 737 series, see the Boeing documentation.

The CFM56-3 (used on the 737-400) can produce exhaust gas temperature (EGT) of up to 930 C at takeoff. This will mix with the cooler bypass air (and ambient air) to reach cooler temperatures further behind the engine. Military jet engines have a smaller bypass ratio, which means there is less of the cool bypass air in the exhaust to mix with the hot core exhaust.

The 737-800 chart makes no sense. First, the axes are the wrong way around -- you choose the distance behind the tail and measure the exhaust velocity there, so the distance should be on the horizontal axis and velocity on the vertical. Second, the small secondary peak at velocity ~230 means that the graph claims three different velocities at ~30m aft (about 180, 220 and 240). Third, a 737 at take-off thrust really generates 60kph winds half a kilometer behind itself? That seems infeasibly high, given the turbulent flow.
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David RicherbyJul 2 '14 at 18:55

@DavidRicherby I agree that the axes should be switched. The peak around 230m is due to the function chosen to connect the data points. However, the values do seem to match the Boeing data. The velocities do seem high, but this is why they have blast fences. Just watch that linked video: those engines are 3 times as powerful as a 737 engine, but still have high velocity blast at least 100m behind the engines.
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foootJul 2 '14 at 19:19

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Those Lyulka 31 engines have a much lower bypass ratio, hence a much higher exhaust speed, than any of the 737 engines you mention. OTOH, the mass flow is lower, so the jet blast is smaller in diameter but more intense.
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Peter KämpfJul 2 '14 at 19:26